The present invention relates to gas turbine engines. In particular, the invention relates to adjusting an impeller blade clearance of a radial compressor in a gas turbine engine.
Gas turbine engines generally comprise a compressor and a turbine. Smaller gas turbines often employ a centrifugal or radial compressor, due to its inherent space efficiency. The primary component of a radial compressor is a compressor impeller. The compressor impeller compresses incoming air which is directed through a diffuser to a combustion chamber, mixed with fuel and ignited. The turbine is propelled by rapidly expanding gases resulting from the combustion of the fuel and the compressed incoming air. The compressor impeller is linked to, and powered by, the turbine.
Overall gas turbine engine efficiency is determined in part by a compression ratio (air pressure exiting the compressor divided by the air pressure entering the compressor). The higher the compression ratio, the higher the gas turbine engine efficiency. The compression ratio is a function of the efficiency of the compressor. The efficiency of a radial compressor is strongly associated with a radial clearance between blade tips of a compressor impeller and a compressor shroud radially surrounding the compressor impeller. As engine and environmental conditions change over the operating range of the engine, this radial clearance varies from a relatively large clearance to no clearance at all. Under conditions resulting in a relatively large clearance, air leaks past the blade tips resulting in a reduction of the compression ratio and a loss of compressor efficiency. Under conditions leading to no clearance at all, the blade tips may rub against the compressor shroud. Such blade rubbing not only reduces compressor efficiency, but may also damage the compressor impeller. Thus, compressor efficiency, and ultimately gas turbine engine efficiency relies in part on maintaining a relatively small radial clearance between blade tips of the compressor impeller and the compressor shroud, while ensuring the radial clearance is sufficient to prevent blade rubbing.